The present invention relates to a dual charge pump system for use in an electronic musical instrument, and in particular to an envelope generator using such a circuit to generate a keying envelope having exponential attack and decay.
Electronic musical instuments, such as electronic organs, employ keyers for connecting tones produced by a tone generator to the output circuitry of the organ in accordance with depressed keys on the keyboard. The individual keyers, which may be dedicated to a single key or footage or shared among all of the tones playable by the organ on an assignment basis, typically have a tone input connected to a tone generator and an envelope input connected to an envelope generator, which in turn is controlled by depressing a key of the keyboard. Since the control voltage produced in response to the depression of a key is generally a DC level which changes almost instantaneously from the key released voltage level to the key depressed voltage level or vice versa, it is necessary to convert this abrupt change in voltage to an exponential curve having an attack and decay, respectively, characteristic of the instrument or voice selected.
A method commonly used to control the rate of attack or decay of a keyer control signal employs a capacitor-resistor network having a desired time constant. The keyer control signal is obtained by the voltage impressed upon the capacitor, which will charge or discharge at a controlled rate, as determined by the parameters of the network. With the advent of semiconductor technology and the ability to integrate thousands of circuit elements on a single chip, the electronic organ industry has moved rapidly to place greater and greater portions of the electronic circuitry for the organ on integrated circuit chips. In the past, however, controlling the keyer control signals by the use of a capacitor-resistor network has required the use of capacitors external to the chip because of the large capacitor values required.
Subsequently, keyer systems have been developed wherein the equivalent of the RC envelope generation circuit can be completely integrated through the use of a charge pump circuit. In U.S. Pat. No. 4,205,581 such an envelope generation circuit is disclosed wherein the abrupt level change of the keying voltage is converted to an exponential curve by the use of a pair of electronic switches and a pair of capacitors. Electronic switches, which can be field effect transistors or other bidirectional electronic switches, are connected in series and one of the capacitors is connected to the common terminal of the transistors and the second capacitor is connected to the second terminal of one of the transistors with the opposite sides of the capacitors connected to a reference potential, such as ground. Each of the transistors is provided with a gate terminal which controls the impedance of the transistor, and gating signals are supplied to the gates thereof alternatively with the frequency of the gating signals determining the relative time constant established from the initial onset of the keyer activating signal. Also controlling the time constant is the ratio between the two capacitors, which is generally quite large, such as 90:1. The control voltage developed on the second capacitor is connected to the keyer envelope input which may be, for example, the gate terminal of a field effect transistor forming the keyer.
The charge at the input of the envelope generator is immediately transferred to the first capacitor when the first series electronic switch conducts. On the next half cycle, the first switch is turned off and the second switch conducts thereby transferring charge from the first capacitor to the second capacitor in accordance with the ratio of the capacitors. Thus, the voltage at the input and on the first capacitor is incrementally transferred to the second capacitor as the electronic switches are rapidly clocked so that after a given number of clock cycles, the voltage on the second capacitor is the same as that on the first capacitor and on the input of the envelope generator. This voltage, which rises or falls exponentially in discrete steps, causes keying of the tone signal at the other input of the keyer to increase or fall in amplitude correspondingly thereby producing the characteristic attack or decay associated with organ tones. U.S. Pat. No. 4,205,581 is expressly incorporated herein by reference.
Although the use of this type of charge pump in a keyer results in substantial savings because of the ability to completely integrate it, the fact that the wave form changes from its minimum to its maximum or vice versa in a plurality of discrete steps rather than as a smooth wave as was the case with discrete RC circuits, introduces audible distortion. This distortion appears as a signal at the frequency of the steps and introduces a twang into the keyed tone. Although this distortion can be reduced by reducing the amplitude changes of the individual steps, this requires a larger capacitance ratio. There is an upper limit to the ratio of capacitors which can be integrated, however, and it has not proven feasible to increase this ratio to the point where the audible distortion can be reduced to a satisfactory level.